Previous research divided long bone growth into intramembranous and endochondral ossification. The latter contributes to longitudinal bone growth through the addition of new bone tissue at the physeal growth plate. Bones, also, increase in thickness through appositional bone growth of the cortical bone. The aim of the present study was to investigate the age-related histomorphometrical changes that occur during the growth of the humerus in rats as a model of the long bone. Samples were collected for light microscopy from the right humerus at 3, 4, 6, 8, 10, 13, and 20 weeks of age. Histological investigation revealed that the distal physeal growth plate could be detected at 3 weeks of age, while at 4 weeks of age it was replaced by bone trabeculae. Morphometrically, differences were observed in the thickness of the proximal physeal growth plate, specifically its proliferative zone. The thickness of the proliferative zone initially decreased, then showed a significant increase around the age of puberty (6–8 weeks), then decreased significantly from 8–20 weeks of age. The mid-shaft cortical bone thickness showed a significant increase during the study. In conclusion, ageing affected both longitudinal and appositional growth, which both participated in the growth and development of the rat's humerus.